I am Sam Windell, composer and the creator of the website FrozenPlain. The site hosts my sample libraries and Kontakt scripts, which focus on atmospheric instruments. Over the past month I have been immersed in the world of impulse responses creating a library of IRs called Frostfall.

Convolution reverb is an excellent tool for emulating real spaces, it is employed by many. Most of the impulse responses about are directly recorded from halls and rooms using sine sweeps and microphones. With Frostfall, I have taken a slightly different approach. In this article I hope to show how it was made and my general experiences with impulse responses (IRs) and convolution reverberation.

Noise versus pitch

Any sound can either be a noise, or it can be pitched. There is definitely some crossover between these two, but recognizing this was a good starting point for me when making IRs. A pitched sound has constant resonant frequencies. For example, if you were to play an A4 on your piano and look at the frequency spectrum, you would see a large constant peak at 440Hz. In contrast, if you clapped your hands and looked at the frequencies of that, you would see a whole bunch of frequencies, with no particular fundamental one.

The reason that this distinction is important to make is because of how convolution reverbs work.  A good way of thinking about it is that the input audio is multiplied by the IR. Imagine the impulse was the piano note A4, if the audio going into the plugin was also an A, then the frequencies (especially 440Hz) will multiply up to make a crazy loud and unpleasant sound.

So right off the bat, I knew I was looking for interesting noises, and not pitched sounds.

Creating the ‘Natural’ impulses

For a while I had been recording and collecting samples using a Zoom H6, in particular the X-Y stereo mics. Being able to capture sounds on-the-go is great and I’d highly recommend it. Whilst out and about, I was always looking for interesting noises, such as: machine buzz, creaky wood, water, etc. I recorded as much as I could, and then cut the audio down later. I tried to keep the stereo recordings quite wide (the X-Y mics were set at an angle of 120o apart), as I felt this made for a more atmospheric and ambient sound. Some of the recordings had background noise, but unlike conventional sampling, this can actually sound great when used as an impulse.

These dry recordings were then put into Edison audio editor, which is where most of the shaping of the sound happened. The majority of captured sounds were not actually appropriate for use as an IR, but those not eliminated were treated as follows:

  • High pass at about 50Hz – anything below was just muddy rumble.
  • Volume envelopes were drawn to shape the sound over time, sometimes emphasizing transients, or just determining the IR length.
  • It was then saved as a WAV file and tested.
The orange line shows the volume envelope.

The orange line is an example of a potential volume envelope.

Synthetizing impulses

As any producer knows, you can spend a lot of time perfecting a beautiful synth patch, intricately tweaking all of the parameters. Making impulses is different though. My technique was this: if I knew something wouldn’t make a nice sound, I would totally do it.

This resulted in lots of modulation, lots of white noise, and just about anything I could edit and randomize, I would. I used soft synths for this and for the whole time, I would record the output. Each sound was probably made slightly differently, with different synths and effects. Once you are in the noise making mind-set, you can easily get carried away and have no idea how the sound started!

Using my recordings in granular synths was also something I deployed. The Mangle is a great plugin for granular sound design.

The impulses in the White Noise folder were made in the same way; however, the original sound source was always noise. Just as I had done with the natural samples, the audio was high passed and shaped with envelopes.

Formant Impulses

Formants are an interesting topic; they are essentially the frequency peaks of the human voice. To create vowel sounds, formant filters are often used. However, as you know, convolution reverbs work by essentially multiplying the audio by the IR, so you can almost use them as a filter. I therefore began experimenting with EQing and filtering white noise and using that as an IR. The result was a slightly voicey reverb sound, which was an interesting tone for sound design.

My next idea was to make ‘real’ formant IRs. I recorded multiple takes of people making vowel sounds without using their vocal chords, just like whispering. I then layered these sounds together and made a ‘choir’ of ah’s, oo’s, and ee’s. If you use a plugin that has a stretch IR control, you can create some great sounds by pitch shifting these formants.

Frostfall is a pack of 113 impulse responses made using some of the techniques above, if you’re interested in using reverb for sound design and atmosphere, you can pick it up on my website.

– Sam